Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230329145330.23191-1-ivan.orlov0322@gmail.com> <20230329145304.66add47ba9b9fafb71b1e13d@linux-foundation.org>
In-Reply-To: <20230329145304.66add47ba9b9fafb71b1e13d@linux-foundation.org>
From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 29 Mar 2023 17:14:23 -0700
Message-ID: <CAHbLzko=aWBPk5a1P2fgR9yAgc52WG-HX_OBV8=iWFy_D4_eNQ@mail.gmail.com>
Subject: Re: [PATCH] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan_file
To:     Andrew Morton <akpm@linux-foundation.org>,
        "Zach O'Keefe" <zokeefe@google.com>
Cc:     Ivan Orlov <ivan.orlov0322@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, himadrispandya@gmail.com,
        skhan@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, Mar 29, 2023 at 2:53=E2=80=AFPM Andrew Morton <akpm@linux-foundatio=
n.org> wrote:
>
> On Wed, 29 Mar 2023 18:53:30 +0400 Ivan Orlov <ivan.orlov0322@gmail.com> =
wrote:
>
> > Syzkaller reported the following issue:
> >
> > ...
> >
> > The 'xas_store' call during page cache scanning can potentially
> > translate 'xas' into the error state (with the reproducer provided
> > by the syzkaller the error code is -ENOMEM). However, there are no
> > further checks after the 'xas_store', and the next call of 'xas_next'
> > at the start of the scanning cycle doesn't increase the xa_index,
> > and the issue occurs.
> >
> > This patch will add the xarray state error checking after the
> > 'xas_store' and the corresponding result error code.
> >
> > Tested via syzbot.
> >
> > Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
> > Link: https://syzkaller.appspot.com/bug?id=3D7d6bb3760e026ece7524500fe4=
4fb024a0e959fc
> > Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
> > ---
> >  mm/khugepaged.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 92e6f56a932d..4d9850d9ea7f 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -55,6 +55,7 @@ enum scan_result {
> >       SCAN_CGROUP_CHARGE_FAIL,
> >       SCAN_TRUNCATED,
> >       SCAN_PAGE_HAS_PRIVATE,
> > +     SCAN_STORE_FAILED,
> >  };
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, u=
nsigned long addr,
> >                                       goto xa_locked;
> >                               }
> >                               xas_store(&xas, hpage);
> > +                             if (xas_error(&xas)) {
> > +                                     /* revert shmem_charge performed
> > +                                      * in the previous condition
> > +                                      */
> > +                                     mapping->nrpages--;
> > +                                     shmem_uncharge(mapping->host, 1);
> > +                                     result =3D SCAN_STORE_FAILED;
> > +                                     goto xa_locked;
> > +                             }
> >                               nr_none++;
> >                               continue;
> >                       }
>
> Needs this, I assume.
>
> --- a/include/trace/events/huge_memory.h~mm-khugepaged-fix-kernel-bug-in-=
hpage_collapse_scan_file-fix
> +++ a/include/trace/events/huge_memory.h
> @@ -36,7 +36,8 @@
>         EM( SCAN_ALLOC_HUGE_PAGE_FAIL,  "alloc_huge_page_failed")       \
>         EM( SCAN_CGROUP_CHARGE_FAIL,    "ccgroup_charge_failed")        \
>         EM( SCAN_TRUNCATED,             "truncated")                    \
> -       EMe(SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> +       EM( SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> +       EMe(SCAN_STORE_FAILED,          "store_failed")

I'm a little bit reluctant to make the error code list longer, can we
just return SCAN_FAIL? IIUC this issue should happen very rarely,
maybe not worth a new error code.

Basically the rollback approach makes sense to me. IIRC Zach was
looking into the same problem, loop him in. He may share some
thoughts.

>
>  #undef EM
>  #undef EMe
> _
>
>